1. Field of the Invention
The present invention relates to a digital television (DTV) reception apparatus, and more particularly, to a synchronization signal detection circuit of a DTV receiver.
2. Description of the Related Art
In recent years, the digitalization of broadcast television has been accelerated. Digital broadcast systems developed in Europe have been standardized under Terrestrial Digital Video Broadcasting (DVB-T), which is a standard of digital broadcasting based on orthogonal frequency division multiplexing (OFDM), while digital broadcast systems developed in the United States have been standardized under Vestigial Sideband (VSB). The Advanced Television SubCommittee (ATSC), which is an organization that defines standards for DTV components, has specified the vestigial side band (VSB) signal necessary for transmission of a DTV signal. For those countries which have adopted U.S. DTV reception systems, it is necessary to develop robust DTV reception apparatuses (e.g., DTV receivers and DTV set-top boxes) that can reliably demodulate and decode VSB signals having considerable amounts of noise in a multi-path channel environment.
FIG. 1 illustrates the structure of a standard VSB data frame that is transmitted to and received by a DTV receiver. Referring to FIG. 1, each data frame of the VSB signal is consists of two fields, i.e., an odd field and an even field. Each of the two fields is consists of 313 segments, with a first segment corresponding to a field sync signal. The first segment of each of the odd and even fields (labeled as Field sync #1 and Field synch #2, respectively) contains a field synchronization (field sync) signal (a data-field-synchronization (DFS) signal), and the subsequent segments of each of the odd and even fields contain Data and forward error correction (FEC) codes. Each of the segments of each of the odd and even fields has 832 symbols. The first four symbols of each of the segments of each of the odd and even fields contain a segment synchronization signal (4-symbol data-segment-synchronization (DSS)) sequence generated at levels of +5, −5, −5, and +5.
FIG. 2 is a diagram illustrating the structure of the field synchronization signals in the first segment in each field of the VSB signal of FIG. 1. Referring to FIG. 2, the field sync signal in the first segment of each field also contains a total of 832 symbols, with the first four symbols corresponding to a segment sync signal. In order to make the VSB signal more receivable, training sequences are embedded into the first segment (containing the field sync signal) of each of the odd and even fields of the VSB signal. The field synchronization signal includes four pseudo-random training sequences for a channel equalizer: a pseudo-random number (PN) 511 sequence, comprised of 511 symbols; and three PN63 sequences, each of which is comprised of 63 symbols. The sign of the second PN63 sequence of the three PN63 sequences changes whenever a field changes, thereby indicating whether a field is the first (odd) or second (even) field of the data frame. A synchronization signal detection circuit (see FIG. 3) determines the profile of the amplitudes and positions (phase) of received multi-path signals, using the PN511 sequence, and generates a plurality of synchronization signals necessary for various DTV reception operations, such as a decoding operation, using the synchronization signals.
FIG. 3 is a block diagram of a conventional DTV receiver 300. Referring to FIG. 3, the conventional DTV receiver 300 includes a tuner 310, a demodulator 320, a noise rejection filter (NRF) 330, a synchronization signal detection circuit 340, and a (channel) equalizer 350. The demodulator 320, which demodulates a received converted digital signal into a demodulated digital signal, includes an analog-to-digital converter 321, a filtering and down-sampling unit 322, a symbol timing recovery unit 323, a carrier recovery unit 324, and a direct current (DC) removal unit 329. The filtering and down-sampling unit 322 includes a poly-phase filter 325, a 5.38 MHz down-sampler 326, a matched filter 327, and 2.69 MHz sorter 328. The demodulated digital signal output from the demodulator 320 is input to an equalizer 340 through the noise rejection filter (NRF) 330, and then passes through a forward error correction (FEC) unit (not shown) and is then output from the DTV receiver 300 (to a signal processor, not shown). The demodulated digital signal RD output from the demodulator 320 is also input to the synchronization signal detection circuit 340 that detects the frame sync signals from the demodulated digital signal RD, which is output from the demodulator 320.
An imaginary component signal IMAGINARY output from the filtering and down-sampling unit 322 is input to the carrier recovery unit 324. A real component signal REAL is input to the DC removal unit 329 and to the symbol timing recovery unit 323. The DC removal unit 329 generates a real component signal RD by removing any DC component from the real component signal REAL received from the filtering and down-sampling unit 322.
Because there may be obstacles and reflectors in the wireless DTV propagation channel, the transmitted signal arrivals at the DTV receiver from various directions over a multiplicity of paths. This phenomenon is called multipath. It is an unpredictable set of received distortion filters and/or direct signals, each with its own degree of (amplitude) attenuation and (phase) delay. Thus, multipath will cause amplitude and phase fluctuations, and time delay in the received multipath signals. The multipath phenomenon also affects analog television signals (e.g., received by analog NTSC TV sets) resulting in ghosts: multiple images shifted laterally. Ghosting is one of major causes deteriorating analog television picture quality. The ghost appears as a faint image at a laterally shifted position superimposed in the original picture in the television screen, and is may be caused by a multiple path channel by a reflecting body such as a building, mountain, and airplane.
Ghosts are a problem in digital television (DTV) transmissions as well as in NTSC analog television transmissions, although the ghosts are not seen as such by the viewer of the image televised by DTV. Instead, the ghosts cause errors in the data-slicing procedures used to convert symbol coding to binary code groups. If these errors are too frequent in nature, the error correction capabilities of the DTV receiver are overwhelmed, and there is catastrophic failure in the television image. If such catastrophic failure occurs infrequently, it can be masked to some extent by freezing the last transmitted good TV images, such masking being less satisfactory if the TV images contain considerable motion content. The catastrophic failure in the television image is accompanied by loss of sound. Thus, for DTVs the result of multipath interference can be an unusable signal, even though the main path signal may be strong.
Multipath DTV signals other than (e.g., received before or after) a main (e.g., “peak”) received DTV signal are sometimes referred to as “ghost” signals, or pre-ghost signals and post-ghost signals (see e.g., FIG. 5). There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Pre-ghosts can be displaced as much as 6 microseconds from the “main” multipath signal, but typically displacements are no more than 2 microseconds. The ability to defeat multipath interference and pull in DTV stations' transmissions remains central to the evaluation of any DTV receiver.
The synchronization signal detection circuit 340 determines a profile of multi-path signals by calculating correlations among the PN (equalizer training) sequences based on the real component signal RD output from the demodulator 320. Thereafter, the synchronization signal detection circuit 340 selects one of the multi-path signals as the main path signal and generates a synchronization locking control signal LOCK and a plurality of synchronization signals (such as a field synchronization signal and a segment synchronization signal).
The synchronization signal detection circuit 340 of the conventional DTV receiver 300 also decides whether to generate the synchronization locking control signal LOCK. The detected position of the main path signal may vary according to the phase offset of the multi-path signals, and in the case of a loss of the main path signal in a dynamic channel, the synchronization locking control signal LOCK is disabled so that the equalizer 350 stops operating. Additionally, the synchronization signal detection circuit 340 selects a signal corresponding to one of a pre-ghost position, a peak value position, and a post-ghost position in the profile of the multi-path signals as a main path signal. However, the performance of the equalizer 350 is considerably affected by not only signals on the left (pre) and right (post) sides of signals having the peak value but also by a number of other signals adjacent to the multi-path signal having the peak power value. The synchronization signal detection circuit 340 may not be able to detect a dynamic variation of the position of a main path signal and may unnecessarily disable the synchronization locking control signal LOCK so that the equalizer 350 stops operating. The synchronization signal detection circuit 340 determines that the one of the received multi-path signals (e.g., one of A, B, C, D, and E of FIG. 5) that has the greatest (peak) value, (e.g., the multi-path signal B of FIG. 5), as the main path signal without considering the influence on the performance of the equalizer 350. Thus, the synchronization signal detection circuit 340 may adversely affect the operating speed and performance of the equalizer 350, causing the performance of the conventional DTV receiver 300 to deteriorate. Thus, the synchronization signal detection circuit 340 is not optimal for such a dynamic channel environment, and its use may considerably deteriorate the performance of the conventional DTV receiver 300.